Historically, simple backlight devices included only three main components: light sources or lamps, a back reflector, and a front diffuser. Such systems are still in use for general purpose advertising signs and for indoor lighting applications.
Today, backlight devices and other extended area sources come in a wide variety of designs, some of which include many additional components besides the three mentioned above. The growth in the backlight industry has been due largely to the growth in the consumer electronics industry for products that incorporate liquid crystal displays (LCDs), such as computer monitors, television monitors, mobile phones, digital cameras, pocket-sized MP3 music players, personal digital assistants (PDAs), and other hand-held devices.
Although some of these consumer products can use ordinary ambient light to view the display, most include a backlight to make the display visible. In the case of LCD devices, this is because an LCD panel is not self-illuminating, and thus is usually viewed using an illumination assembly or backlight. The backlight is situated on the opposite side of the LCD panel from the viewer, such that light generated by the backlight passes through the LCD to reach the viewer. The backlight incorporates one or more light sources, such as cold cathode fluorescent lamps (CCFLs) or light emitting diodes (LEDs), and distributes light from the sources over an output area that matches the viewable area of the LCD panel. Light emitted by the backlight desirably has sufficient brightness and sufficient spatial uniformity over the output area of the backlight to provide the user with a satisfactory viewing experience of the image produced by the LCD panel.
LCD panels, because of their method of operation, utilize only one polarization state of light, and hence for LCD applications it may be important to know the backlight's brightness and uniformity for light of the correct or useable polarization state, rather than simply the brightness and uniformity of light that may be unpolarized. In that regard, with all other factors being equal, a backlight that emits light predominantly or exclusively in the useable polarization state is more efficient in an LCD application than a backlight that emits unpolarized light. Nevertheless, backlights that emit light that is not exclusively in the useable polarization state, even to the extent of emitting randomly polarized light, are still fully useable in LCD applications, since the non-useable polarization state can be easily eliminated by an absorbing polarizer provided at the back of the LCD panel.
Backlights can be considered to fall into one of two categories depending on where the internal light sources are positioned relative to the output area of the backlight, where the backlight “output area” corresponds to the viewable area or region of the display device. The “output area” of a backlight is sometimes referred to herein as an “output region” or “output surface” to distinguish between the region or surface itself and the area (the numerical quantity having units of square meters, square millimeters, square inches, or the like) of that region or surface.
The first category is “edge-lit.” In an edge-lit backlight, one or more light sources are disposed—from a plan-view perspective—along an outer border or periphery of the backlight construction, generally outside the area or zone corresponding to the output area. Often, the light source(s) are shielded from view by a frame or bezel that borders the output area of the backlight. The light source(s) typically emit light into a component referred to as a “light guide,” particularly in cases where a very thin profile backlight is desired, as in laptop computer displays. The light guide is a clear, solid, and relatively thin plate whose length and width dimensions are on the order of the backlight output area. The light guide uses total internal reflection (TIR) to transport or guide light from the edge-mounted lamps across the entire length or width of the light guide to the opposite edge of the backlight, and a non-uniform pattern of localized extraction structures is provided on a surface of the light guide to redirect some of this guided light out of the light guide toward the output area of the backlight. Such backlights typically also include light management films, such as a reflective material disposed behind or below the light guide, and a reflective polarizing film and prismatic BEF film(s) disposed in front of or above the light guide, to increase on-axis brightness.
The second category is “direct-lit.” In a direct-lit backlight, one or more light sources are disposed—from a plan-view perspective—substantially within the area or zone corresponding to the output area, normally in a regular array or pattern within the zone. Alternatively, one can say that the light source(s) in a direct-lit backlight are disposed directly behind the output area of the backlight. A strongly diffusing plate is typically mounted above the light sources to spread light over the output area. Again, light management films, such as a reflective polarizer film, and prismatic BEF film(s), can also be placed atop the diffuser plate for improved on-axis brightness and efficiency.
In some cases, a direct-lit backlight may also include one or some light sources at the periphery of the backlight, or an edge-lit backlight may include one or some light sources directly behind the output area. In such cases, the backlight is considered “direct-lit” if most of the light originates from directly behind the output area of the backlight, and “edge-lit” if most of the light originates from the periphery of the output area of the backlight.